A digital video cassette recorder employing a helical scanning format has been proposed by a standardization committee.
The proposed standard specifies digital recording of standard definition SD television signals, for example, NTSC or PAL, and high definition television signals having an MPEG compatible structure, such as a proposed Grand Alliance or GA signal. The SD recorder utilizes a compressed component video signal format employing intra field/frame DCT with adaptive quantization and variable length coding. The SD digital VCR or DVCR may digitally record either NTSC or PAL television signals and has sufficient data recording capability to record an advanced television signal.
A specification of the GA signal is included in a draft specification document titled Grand Alliance HDTV System Specification, published in the 1994 Proceeding of the 48th Annual Broadcast Engineering Conference Proceedings, Mar. 20-24 1994. The GA signal employs an MPEG compatible coding method which utilizes an intra-frame coded picture, termed I frame, a forward predicted frame, termed a P frame and a bidirectionally predicted frame, termed a B frame. These three types of frames occur in groups known as GOPs or Groups Of Pictures. The number of frames in a GOP is user definable but may comprise, for example, 15 frames. Each GOP contains one I frame, which may be abutted by two B frames, which are followed by a P frame.
In an analog consumer VCR, "Trick Play" or TP features such as picture in forward or reverse shuttle, fast or slow motion, are readily achievable, since each recorded track typically contains one television field. Hence, reproduction at speeds other than standard, may result in the reproducing head, or heads, crossing multiple tracks and recovering recognizable picture segments. The picture segments may be abutted and provide a recognizable and useful image. An advanced television or MPEG like signal may comprise groups of pictures or GOPs. The GOP may, for example, comprise 15 frames and each frame may be recorded occupying multiple tracks on tape. For example, if 10 tracks are allocated to each frame, then a 15 frame GOP will comprise 150 tracks. During play speed operation, I frame data is recovered which enables the decoding and reconstruction of the predicted P and B frames. However, when a DVCR is operated at a non-standard reproduction speed, the replay heads transduce sections or segments from the multiple tracks. Unfortunately these DVCR tracks no longer represent discrete records of consecutive image fields. Instead these segments contain data resulting mainly from predicted frames. However, since predicted P and B frames require preceding data to facilitate decoding the possibility of reconstructing any usable frames from the reproduced pieces of data is greatly diminished. In addition the MPEG data stream is particularly unforgiving of missing or garbled data. Thus to provide "Trick Play" or non-standard speed replay features requires that specific data be recorded, which when reproduced in a TP mode, is capable of image reconstruction without the use of adjacent or preceding frame information. The specific data, or "Trick Play" data must be semantically correct to allow MPEG decoding. In addition, a selection of "Trick Play" speeds, may require different TP data derivation and may require TP speed specific recorded track locations.
To be capable of reconstruction without preceding frame data requires that "Trick Play" specific data be derived from I frames. The "Trick Play" specific data must be syntactically and semantically correct to allow decoding, for example, by a GA or MPEG compatible decoder. In addition the "Trick Play" or TP data must be inserted into the MPEG like data stream for recording together with the normal play, MPEG like signal. This sharing of the recording channel data capacity may impose constraints in terms of TP data bit rate which may be provided within the available track capacity. The TP data bit rate may be variously utilized or shared between spatial and or temporal resolution in the derived or reconstructed TP image.
Reproduced "Trick Play" image quality may be determined by the complexity of the TP data derivation. For example, a consumer DVCR must derive TP data during recording, essentially in real-time and with only nominal additional data processing expense added to the DVCR cost. Thus real-time consumer DVCR "Trick Play" image quality may appear inferior to TP image data derived by non-real time image processing utilizing sophisticated digital image processing. With non-real time TP image processing for example, an edited program may be processed, possibly on a scene by scene basis, possibly at non-real-time reproduction speeds, to enable the use of sophisticated digital image processing techniques. Such non-real time processing may inherently provide higher quality "Trick Play" images than that attainable with real time processing.